Modeling Local Bubble analogs

II. Synthetic Faraday rotation maps

¹ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ Istituto di Astrofisica e Planetologia Spaziali (IAPS). INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, D-69120 Heidelberg, Germany
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, F-75005 Paris, France

^⋆ Corresponding author.

Received: 12 July 2024
Accepted: 24 April 2025

Abstract

Context. Faraday rotation describes the change in the linear polarization angle of radiation passing through a magnetized plasma. The Faraday rotation is quantified by the rotation measure (RM), which is related to the line-of-sight (LOS) magnetic field component and the thermal electron density traversed by light along its path toward the observer. However, it is challenging to disentangle the signal from different LOS portions and separate the contribution from the local interstellar medium (ISM). As the Solar System is located within the Local Bubble, a low-density, hot cavity formed by past supernova events, it essential to investigate how this environment may impact the observed RM values.

Aims. The present study investigates the imprint of the local environment on the synthetic RM signal, as measured by an observer within a Local Bubble-like cavity. The RM derived from diffuse polarized synchrotron radiation produced by cosmic ray (CR) electrons at decimeter wavelengths is also analyzed.

Methods. We produce synthetic Faraday rotation maps for an observer placed inside a Local Bubble analog, selected from a magnetohydrodynamic (MHD) simulation that resembles the properties of the ISM in the solar vicinity. Using the capabilities of the radiative transfer code POLARIS, we study the imprint of the cavity walls on the RM signal. As the MHD simulation does not account for CR diffusion, we develop a CR toy model to study the Faraday rotation of the diffuse polarized synchrotron radiation.

Results. We find that (i) the imprint of local structures, such as the walls of the Local Bubble analog and the edges of other supernova-blown cavities, is of fundamental importance for interpreting the global Faraday sky; (ii) the Local Bubble has a nonnegligible contribution to the sinusoidal patterns of RM as a function of galactic longitude seen in observations; and (iii) the RM signal from diffuse synchrotron emission shows a strong correspondence with the RM signal generated by the Local Bubble analog walls.